7. REPORTS AND MISCELLANEOUS 



255 



second, the report shall state the frequency, 

 or frequencies, used; the areas where each 

 non-standard frequency was used; and the 

 method employed to determine appropriate 

 frequencies. 



Verification of hydrographic smooth sheets 

 occasionally reveals inconsistencies in hy- 

 drography which are caused by improper use 

 of available correction data. The results of 

 successive bar checks should not be averaged 

 when they differ appreciably (see 5-115). 

 When the phasing head on 808 Fathometers 

 is not shifted correctly, phasing errors 

 change. Results of inconsistent phase com- 

 parisons should not be averaged. 



7-27 Electronic systems calibration re- 

 ports. — When an electronic distance measur- 

 ing system is used to control hydrographic 

 surveys, a report on calibration of the sys- 

 tem and summary of corrections to distance 

 measurements shall be submitted at the end 

 of a project or field season. The methods used 

 to calibrate the equipment should be described 

 briefly ; unusual methods should be described 

 in detail and the necessity for using non- 

 standard methods should be stated. 



Requirements for calibration of EPI equip- 

 ment are stated in 3-29. The results of tests 

 to determine the effect of the ship's heading 

 should be tabulated (see 3-30) . 



Calibration of Shoran equipment is dis- 

 cussed in 3-38 to 42. It is important that a 

 continuous study of Shoran corrections be 

 conducted during progress of the survey. The 

 final report should contain an analysis of 

 the calibrations. The geographic coordinates 

 of all calibration points should be listed and 

 the dates when calibrations were made at 

 each point should be given (see 3-46) . 



Tables of distance corrections should be 

 included and properly referenced to date and 

 survey ship or launch. Computations of po- 

 sition data and similar material should not 

 be included in the report. 



Difficulties in operation of the equipment, 

 other than routine maintenance problems, 

 should be described. 



Corrections to Raydist measurements are 

 described in 3-56 and procedures for cali- 

 brating the equipment are described in 3-58 



to 60. A report on calibration of Raydist 

 equipment shall be submitted at the end of a 

 field season or on completion of a project. 

 The report shall summarize the procedures 

 used and the results obtained. 



When any survey is plotted by boat- 

 smooth sheet methods and an electronic dis- 

 tance measuring system is used for control, 

 the report on calibrations shall include a 

 statement regarding the adequacy of the 

 preliminary calibration data used to correct 

 observed distances. 



7-28 Velocity of sound computations. — 



If the temperature and salinity of sea water 

 at a specific depth are known, the velocity of 

 sound at that depth may be derived from 

 Tables 14, 15, and 16. These tables are based 

 on Tables of the Velocity of Sound in Pure 

 Water and Sea Water, H.D. 282, published 

 by the Hydrographic Department of the 

 British Admiralty. In Table 14 are given 

 the velocities (in meters per second) for 

 various temperatures at surface atmospheric 

 pressure and at an assumed salinity of 35.0 

 "'/oo- The correction to be applied for any 

 other salinity is given in Table 15; it is 

 subtractive for salinities less than 35.0 "/oo 

 and additive for salinities above 35.0 **/oo- 

 The correction for pressure at latitude 45° 

 is given in Table 16; it is always additive. 

 The pressure correction varies not only with 

 hydrostatic pressure, but also with gravity 

 and therefore slightly with latitude ; but the 

 mean values in Table 16 are sufficiently ac- 

 curate for all surveying purposes. The pres- 

 sure correction is nearly proportional to 

 depth, and for depths less than 1,000 fath- 

 oms, it may be found with sufficient accu- 

 racy by dividing the depth (in fathoms) by 

 30 — the result being the correction in meters 

 per second. 



7-29 Velocity from diagrams. — The dia- 

 grams (Figs. 87 and 88) may be used in- 

 stead of the tables in 7-28 to find velocities. 

 These diagrams are based on the values in 

 the tables but, owing to their small scale, 

 the velocities cannot be derived with quite 

 the accuracy that the tables will give. But 

 if the values are interpolated carefully from 

 the diagrams the resulting velocities should 



